What’s the Most Obscure Ammo?

So just for the fun of it, I decided to do something different today. So, in the comments, I want readers to name the most obscure ammunition they can think of. I’ll go first. Let’s have some fun. Maybe we’ll even learn something.

A similar situation to .38/.45 Clerke, which is probably known to a lot of people who wanted to have a really fast pistol round in a handgun with a simple barrel change. The slightly weird cousin of .357 Sig and .400 Corbon.